DISCUSSION OF BACKGROUND
Fluid treatment systems typically require one or more valves or valving systems in order to control the fluid flow, turn the flow on and off, and/or change the flow paths between and among components in the fluid system. One fluid system that will become of increasing importance is a system for the separation or purification of hydrogen, so that pure or substantially pure hydrogen can be used as an alternative to conventional fuels such as gasoline. For example, environmental and preservationist concerns have resulted in the gradual rejection of fossil fuels as the primary energy source of the future. Consequently, different methods for implementing the widespread use of hydrogen fuel cells are currently being explored. As used herein, pure or substantially pure hydrogen is intended to mean that the hydrogen is of sufficient purity or the purity intended because, obviously, absolute or perfect purity is not practical.
In a conventional pressure swing adsorption (PSA) system, a five-step process is used to separate hydrogen from a hydrogen-rich feed gas. In the first “adsorption” step, feed gas is passed through a first vessel including adsorbent material, where impurities are selectively adsorbed. Pure hydrogen product exits the vessel at high pressure, and the first vessel, now saturated with impurities, must be regenerated. In the second “co-current depressurization” step, hydrogen trapped in void spaces of the first vessel is directed into another vessel by depressurizing the first vessel in a co-current direction (i.e., in the direction in which the feed gas was originally introduced in the first vessel). In the third “counter-current depressurization” step, depressurization is performed in the first vessel in a counter-current direction (i.e., opposite to the co-current direction), and impurities are transferred to a tail stream. In the fourth “purge” step, the first vessel is cleaned at low pressure using a hydrogen-rich stream obtained from another vessel during co-current depressurization, thereby further transferring impurities into the tail stream. In the fifth “counter-current repressurization” step, the first vessel is repressurized with pure hydrogen product from two other vessels, one vessel undergoing the co-current depressurization step and the other undergoing the adsorption step. As should be apparent, to operate this process, the flow paths or flow relationships are repeatedly changed. If this process is to be implemented on a commercial basis, the provision of a flow system or valving arrangement to repeatedly change the flow relationships in a reliable manner over extended periods of time presents a challenge that existing valve systems do not satisfactorily meet.
FIG. 9 illustrates an example of a gas processing system 52 that can be used for the PSA process described above. The system 52 is linearly-arranged and includes five vessels 53, each of which exclusively undergoes one of the five PSA steps at any given time during operation. Each vessel 53 receives feed fluid via a conduit 54, and each vessel 53 processes the feed fluid to produce a tail (or waste) fluid that is transferred through conduit 55 and a product fluid (the hydrogen fuel product) that is transferred through conduit 56. The vessels 53 are configured to communicate with each other via a conduit 62. Four valves 57 (which could be, e.g., ball valves or butterfly valves) are attached to each vessel 53 to control fluid flow into and out of conduits 54-56. Each valve 57 is controlled by a pneumatic actuator 58, which is in turn powered and controlled by power lines 59 and instrument air pipes 60. In order to properly sequence the gas processing system 52, a complex computer algorithm must be used to control the opening and closing of valves 57.
It is apparent that the processing system 52 requires many components and, consequently, is cumbersome, complex and expensive to build and operate, making such a system 52 undesirable for use in processing fluids, e.g., to obtain hydrogen, in an efficient manner.
Several multi-port systems have been proposed, including the systems described in U.S. Pat. Nos. 4,925,464; 5,814,130; 5,814,131; 5,807,423; and 6,457,485, the disclosures of which are hereby incorporated by reference in their entireties. However, each of these disclosed systems utilize components with complex geometries, which can require expensive manufacturing processes and result in unreliable operation. Accordingly, such designs also are less than optimal, particularly for use in hydrogen purification.
One factor that contributes to the design complexities is that the multi-port valves of these systems are in continuous rotation, requiring large and/or complex apertured plates to control the communication relationships or flow paths for desired time intervals. Because the apertured plates rotate continuously and must maintain a sealed relationship to prevent leakage, the arrangements constantly battle large forces required to maintain a sealed relationship of the relatively large components and the associated torque required to rotate the assembly components.